Power Z axis

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tornitore45
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Power Z axis

Post by tornitore45 »

After 12 years on the mill and 10 year of service as a drill the motor/gearhead gave up. No brushes left.
I replaced the motor/gearhead from another identical drill I salvaged.

This time I thought I will be a bit more scientific about it

My mill has a square head column with dovetail slides for the head
The Z axis crank was grooved to accept a belt. The drill head has a smaller pulley. The belt tensioner is a lever with a ball bearing pulley and a microswitch. With the belt slack I can crank the wheel manually without resistance from the motor. Actuating the belt tensioner starts the 12V supply via the remote start control pin.

I wanted to "measure" the head weight to have an idea of the energy required to lift it. I cranked the head down onto a bathroom scale to 250# then carefully turned the crank until the jackscrew was in its backlash region unloading the screw and the scale slowly dropped the reading to 200#
Right or wrong let's call the head weight + Static friction = 200# = 890 Newtons

Then I measured voltage and current into the motor. What blew my mind is that there no significant difference in power drain going UP or DOWN
Let's call it 63W
It also took exactly 60 second to move 11"= 0.28m

I spare you the high school physic but the conclusion is that to move the head and its friction 11" in 60 seconds requires 4.1W
I am unable to separate the head weight from the friction so we just stab that the head alone take 3.5W while the motor absorbs 63 W
A total efficiency of 5.6% Granted this is not National Bureau of Standard quality metrology but the conclusion is that is practically ALL friction and some electrical losses.
Mauro Gaetano
in Austin TX
kl7sg
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Re: Power Z axis

Post by kl7sg »

You might try approaching this from the gravitational potential energy direction.

This should separate the friction from the head movement energy calculation.
Anyway, I think "Delta U = mg delta z" should give you a better picture of the energy required to lift just the head.

Also, I don't understand why your are using the Static Friction (Stiction) as it is just the break away friction.

Mike
Have a nice day,

Mike
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tornitore45
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Re: Power Z axis

Post by tornitore45 »

m x g x delta z = Weight x delta z = which is what I used
I can not read the electrical Work but can read power. I know the work to raise the head and the time, therefore can calculate the power necessary.
Either I compare power or work is a matter of accounting for time on one side of the equation or the other.

I made an error above, the measured 200# is the head weight MINUS the static friction. The head is probably weighting more than 200# which increase the efficiency calculated above. The combined (Head - Static Friction) Is the only thing I can measure without taking the head off and putting it on a scale.

Admittedly this is not a very precise measure but the conclusion remains that the job (Work) is all fighting frictions, very little Work is done raising the head. This is what makes predicting the required motor power so trial and error. I expected to see a large difference in the electrical power (which I can measure accurately) between Raising and Lowering the head.
Mauro Gaetano
in Austin TX
kl7sg
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Re: Power Z axis

Post by kl7sg »

Delta U = 247.9 joules
Motor energy = 63 x 11 = 693 joules

efficiency = 247.9/693 = 35.7%

The rest of the energy is probably lost in the drive belt, frictional losses and the energy to spin the acme screw.

mike
Have a nice day,

Mike
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tornitore45
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Re: Power Z axis

Post by tornitore45 »

Watt x Inches is NOT Joules
Watt x Seconds = Joules
Mauro Gaetano
in Austin TX
kl7sg
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Re: Power Z axis

Post by kl7sg »

My bad, I read your post incorrectly.

motor energy = 63 x 60 = 3780 Joules.
efficiency = 6.5%

Due to mechanical losses and other factors, The typical efficiency for for such systems runs around 30%.

My opinion: there is probably something wrong with your assumptions or measurements.

63/746= .084 Horsepower.

Possibly the low efficiency is caused by having a very large motor (relative to the power actually required) driving the head lift system or other such overdesign features. Or, maybe simply, the efficiency was not considered a significant factor when designing the system.

Your argument regarding sizing motors is not a problem I have encountered in many years of engineering. The energy required to lift the head is easy to determine. The same goes for energy loss in the belt, frictional losses, bearing losses ...
Have a nice day,

Mike
John Hasler
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Re: Power Z axis

Post by John Hasler »

The acme screw alone is unlikely to exceed 30% efficiency.
kl7sg
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Re: Power Z axis

Post by kl7sg »

:D
Have a nice day,

Mike
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tornitore45
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Re: Power Z axis

Post by tornitore45 »

The acme screw alone is unlikely to exceed 30% efficiency.
That is a very good data point, and it got me reflecting on screws.

The JackScrew is nothing but an inclined plane wrapped around a pole
The last time I was able to see the screw it appear to be a 16mm diameter, can't get to measure it. I know the lead is 0.05"
It take 220 turns to move 11" for a total sliding length of pie D 11/0.05 or about 11 meters
Using a coefficient of friction of 0.084 It requires 75N to push an 890N head up the incline
75 N x 11 m / 60 second = 13.7 Watt
Lifting the head was calculated to be 4.1 Watt

So the efficiency of the JackScrew alone is 4.1/(4.1+13.7) = 23% very much in the ball park considering the uncertainty of the various parameters.
Mauro Gaetano
in Austin TX
kl7sg
Posts: 75
Joined: Thu Oct 08, 2020 12:00 pm
Location: Springtown, TX

Re: Power Z axis

Post by kl7sg »

Exactly Mauro,

Then, if you make the assumption: the drive belt efficiency is approximately equal to 95% you will see that the electric motor is probably not operating in its design power range.

I have attached information on motor efficiency and power screw efficiency.
Electric motor efficiency.pdf
(190.26 KiB) Downloaded 291 times
power-screw-4.gif
Anyway, with a few supported assumptions the size of the motor can be "guessed" fairly well.
Have a nice day,

Mike
kl7sg
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Re: Power Z axis

Post by kl7sg »

Also, the total efficiency is the product of the individual efficiencies.

Jack screw: 23%
Belt: 95%
Motor: 30%

.95 x .23 x .30 = 6.5% over all efficiency
Have a nice day,

Mike
John Hasler
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Re: Power Z axis

Post by John Hasler »

That motor efficiency data is for induction motors. Mauro is using a DC motor. Brushed DC motor efficiency is approximately linear with speed. Brushless DC motors do a bit better.
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